Engine Tuning Theory With : Fuel Injection

[2drSE-i]

Ok heres what i dont understand about anything you guys have said so far. In a book im reading (engine building for dummies, basically haha) it states that 14.7 (stoich) is the most effecient AFR because stoich is the point at which the fuel and air mixture burns most effeciently. Would this not mean that the most power should be found at stoich? With anything over stoich you get too "slow" (if you will) of a burn, and anything leaner than stoick will burn too "quickly" (again, if you will. i should have said colder and hotter...) I don't believe that engines were tuned to stoich (respectively tuned) because of cats or o2 sensors, but because it is the most effecient AFR.

[MessyHonda]

when i got my car tuned i got it tuned with a wideband. and they did a good job that it killed my clutch.

[Accordtheory]

A mixture lean on fuel will react more quickly, giving a sharper rise in both heat and pressure

This is incorrect.. The damn chainsaw thing is a flawed analogy.

[Accordtheory]

I thought that lean backfire was when the fuel is so lean that it detonates before the intake valve closes, hence backfiring through the intake manifold... I don't see how a slower burn could cause this, you'd think if it burned more slowly it wouldn't have time to ignite fully before the intake valve closes.

What..? The lean backfire is caused by the air/fuel still burning on the overlap cycle. That's Proof of the slower burn of a lean mixture. The lean mixture is still burning on the exhaust stroke, popping out the exhaust, if it's bad enough, it's still burning when the intake valve opens on the top of the exhaust stroke, igniting the fresh mixture. That causes an intake backfire.

[Ichiban]

Sorry all, I thought you were refering to "afterfiring" caused by a lean mixture.

What..? The lean backfire is caused by the air/fuel still burning on the overlap cycle. That's Proof of the slower burn of a lean mixture. The lean mixture is still burning on the exhaust stroke, popping out the exhaust, if it's bad enough, it's still burning when the intake valve opens on the top of the exhaust stroke, igniting the fresh mixture. That causes an intake backfire.

Uh, if it goes off in the exhaust during overlap, it will be conducted back through the cylinder and into the intake, would it not?

Mistake I made earlier was that I thought you were talking about the afterfiring that happens as an engine leans out. To be sure, you're talking about the giant fireball out of the carb, right? I always thought that was indicitave of an ignition timing problem, more than a lean mixture.

[Ichiban]

This is incorrect.. The damn chainsaw thing is a flawed analogy.

So then why does a lean mixture burn hot? This is a fact you can't argue with. The analogy is not flawed. It's still an engine.

[Accordtheory]

Sorry all, I thought you were refering to "afterfiring" caused by a lean mixture.



Uh, if it goes off in the exhaust during overlap, it will be conducted back through the cylinder and into the intake, would it not?

Mistake I made earlier was that I thought you were talking about the afterfiring that happens as an engine leans out. To be sure, you're talking about the giant fireball out of the carb, right? I always thought that was indicitave of an ignition timing problem, more than a lean mixture.

Well, I've never hear the term "afterfiring", but I guess in this instance we mean the same thing. To me, a backfire means either flames out the exhaust or the intake. If your ign timing is far enough off to fire the spark plug on the intake stroke, obviously the motor won't run like that, and that's different from what I'm talking about. I'm saying the motor's already running, and if you lean it out and if you don't blow it up first, it will backfire. Esp with forced induction.

[Accordtheory]

So then why does a lean mixture burn hot? This is a fact you can't argue with. The analogy is not flawed. It's still an engine.

First, I Can and Have been arguing against that, all along! I've been saying it's not as hot as stoich, but burns for a longer time. The answer to the chainsaw thing lies in the fact that it bogs when a load is applied to it with that mixture at that rpm. Explain that.

[Accordtheory]

I don't believe that engines were tuned to stoich (respectively tuned) because of cats or o2 sensors, but because it is the most effecient AFR.

You must not have read anything I wrote about dynamic comression, extracting the most energy out of a given pulsewidth, etc. In other words, most of what I wrote.

[Ichiban]

Well, I've never hear the term "afterfiring", but I guess in this instance we mean the same thing. To me, a backfire means either flames out the exhaust or the intake. If your ign timing is far enough off to fire the spark plug on the intake stroke, obviously the motor won't run like that, and that's different from what I'm talking about. I'm saying the motor's already running, and if you lean it out and if you don't blow it up first, it will backfire. Esp with forced induction.

I guess there's confusion around the terms backfiring and afterfiring. Afterfiring happens out of the exhaust (everyone calls incorrectly calls it a backfire) while true backfires happen in the intake only.

I also just did some quick reading (no reputable sources though) that state lean mixtures may either:
-Ignite rapidly and detonate, raising temperatures and causing damage.
-Ignite and burn very unevenly/partially/slowly (apparently if fuel distribution is uneven it can burn in pockets and stay burning for awhile)
-Not ignite at all

So, what do you think, Accordtheory?

[Ichiban]

First, I Can and Have been arguing against that, all along! I've been saying it's not as hot as stoich, but burns for a longer time. The answer to the chainsaw thing lies in the fact that it bogs when a load is applied to it with that mixture at that rpm. Explain that.

Because lean mixtures burn holes in pistons, cook valves, and weld spark plug electrodes together. Google "lean mixture" and see what you get. Talk to anyone with a sled or bike, or any engine tuner. Lean engines run hot. EGT's by cylinder are used on multi carbed engines to tune them. Hot=lean, cold=rich.

Maybe the faster burning or detonating mixture in the chainsaw cylinder can push the non-loaded piston out of the way faster in the free rev situation, but doesn't have the force (torque) to compensate for the load applied. Diesel engines of comparable displacement will produce more torque than a gas engine, but less overall kW, so that idea is totally possible.

[89T]

hmm! you guy's are killing me.
has any one thought about flash points yet?
leaner mix = lower flash point??
detonation?
too much fuel = higher egt's???
hell i wish i could answer all but i cant plain and simple. I am not a tuner, i am a builder.

[Ichiban]

Don't vacuum leaks result in a higher, erratic idle? Guess more air makes engine go faster, and shittier.

[2ndGenGuy]

Would this explain why a rich mixture burns cooler (or a lean mixture burns hotter in relation)?

With a liquid fuel engine, the addition of more fuel also lowers the combustion temperature by the condensing effect. Here the fuel is evaporating and absorbing combustion heat.

[89T]

i think there is alot more to this than what you guy's are focusing on.

2g guy yep that's one reason the target afr for the most efficiant power is found at 12.5:1afr.

[2drSE-i]

You must not have read anything I wrote about dynamic comression, extracting the most energy out of a given pulsewidth, etc. In other words, most of what I wrote.

i did, i just didnt read it three times until after i posted...i wish i had then i wouldn't look so dumb haha

[cygnus x-1]

I know I've read in several places now that the hottest combustion temperature occurs at the stoich ratio, and that richer or leaner mixtures actually result in cooler combustion temps. The only reference I can find at the moment though is here:

http://www.foxvalleykart.com/egt.html

Now with burnt exhaust valves, I'm not sure it's as simple as: hotter combustion=hotter valves. I read an article about this somewhere on the web and now I can't find it. It might have been on team-integra.net. Still searching...

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[Accordtheory]

What sucks is that almost everything I've written here has no direct formal research published anywhere that I can find. I posted some of this on honda-tech, they didn't even touch it. I knew they wouldn't, tuners don't like to talk, but that's okay, because most of them don't know shit anyway. Seriously, there have been so many posts on h-t with tuners who can't even make a motor not blow up, not to mention tune for better fuel economy than stock. If they do have some info, they don't understand what is really going on inside the cylinders, how it works, and can't explain it. Even one of the best tuners on h-t, mase, couldn't explain the best rpm vs bsfc for a given output thing. He didn't even really try, he was just like, f it, I've got school, maybe later.

So far, not one of you guys has even come close to refuting what I've said about leaner isn't hotter than stoich, (except for the chainsaw thing, which I think is flawed and will get into later) but causes engine damage by burning more slowly, still at a high temp. Yet you guys keep arguing with that. Evidence of thermal damage doesn't necessarily correspond to a higher temperature. It's temp vs Time of exposure.

guyhatesmycar, that was an interesting article you PMd me with, I don't know why you didn't put the link on this thread. However, there was a key piece of info lacking from it. Those damn airplanes don't have widebands! The thing I found most interesting was the WOT/LOP egt operation, but I already knew there was something else to it..and there was, altitude. The manifold press was only 75% of atmospheric, and they said engine damage was a definite possibility doing this below 6000ft. I don't know what a/f produces the hottest egt. I'm guessing somewhere around 17, 18:1? After all, it still has to light off in the conventional motor..

As far as specifically how running lean causes engine damage, I haven't gotten into that yet, now I will. Primarily I would say it leads to detonation. There are many misconceptions in this post about lean a/f mixtures and detonation, a lean mix does not have a lower auto ignition point, it is Not more susceptible to detonation in itself. It doesn Not burn more quickly. It burns slower, and it burns hot, but not hotter than stoich, to the best of my knowledge. It is belief that this increased exposure time at high temp is what heats up everything in the cylinder more, esp the spark plug and anything susceptible, like sharp edges in the chamber, and gets that glowing exhaust valve even hotter, since it's hotter passing through it, since it's either still burning or has just stopped, unlike a stoich misture, which has stopped by the time that valve opens. Then I would say this increased heat left behind in the chamber causes the fuel to ignite before the plug fires. This causes the entire a/f charge to autoignite at once (detonation) just like if you advanced your ign timing. What I see happening is that the a/f is trying to axpand while the piston is still rising, and the reduction in chamber volume vs the burning mixture compresses the mixture to where it reaches it's autoignition point, like a diesel, and it explodes at once. If the ign point is later, you don't get the 2 forces fighting each other as much, and the a/f is able to burn like normal.

Just to reiterate, I have never read any of what i just wrote anywhere.

As far as the chainsaw, I am theorizing that the carb is unable to maintain the stoich ratio across its entire operating range. At peak rpm, the venturi becomes a restriction, limiting the rpm, (designed that way) and going rich. So now you close the screw, and it revs higher. But now it doesn't have any power once you put a load on it and drop the rpm, because at that rpm, now it's lean.

[Accordtheory]

I know I've read in several places now that the hottest combustion temperature occurs at the stoich ratio, and that richer or leaner mixtures actually result in cooler combustion temps. The only reference I can find at the moment though is here:

http://www.foxvalleykart.com/egt.html

Now with burnt exhaust valves, I'm not sure it's as simple as: hotter combustion=hotter valves. I read an article about this somewhere on the web and now I can't find it. It might have been on team-integra.net. Still searching...

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That was a decent article. The think about detonation and cylinder head temp is what I learned in an aircraft engine class I took, I guess they haven't heard of knock sensors. Air cooled aircraft engines are like 40 years behind cars.
About egt guages, first, I hate those things. They respond so slowly, not like a wideband, that they're almost meaningless under transient conditions. Second, what they described in that article, where the egt drops under acceleration as being an indicator of detonation, is not what true when it comes to a turbo car, at all. Your egt should drop a little, because you run richer under boost. My car was 1400 cruise, 1300-1350 under boost. But then, you also run less timing, which counters that by increasing the egts, since the combustion is timed to closer to the valve opening. See how worthless the egt guage is? Spend the $ on a wideband..

The exhaust valve could be damaged by anything that increases the egt. Either running lean or with less timing. I would say most valves are damaged by incorrect lash though. If you're bleeding combustion pressure through the valve, that will quickly destroy it.

[Accordtheory]

i think there is alot more to this than what you guy's are focusing on.

2g guy yep that's one reason the target afr for the most efficiant power is found at 12.5:1afr.

Where are you coming up with this? Almost everything I have written is about trying to get the most out of a certain pulsewidth, ie, efficiency, and you're talking about trying to get the most out of a given mass of air, ie, max power. None of us have even really gotten into this yet.

And 12.5:1 isn't even the most efficient for producing power, it's just a usually accepted compromise between chamber cooling and power production. For max power production, the goal is to make sure as much of the oxygen is combined with fuel as possible. This results in running just slightly richer than stoich. However, this usually is not tolerated by the motor, because of the heat production, and the motor starts to knock. Since more power can be preserved by adding fuel than subtracting timing, fuel is added.

I suppose we could take this discussion in another direction, and start talking about tuning for power..

I need to get my damn car running so I can start experimenting firsthand and blow it up..

[cygnus x-1]

More about lean mixtures and EGT...

http://www.sdsefi.com/techegt.htm
http://www.hydrogen-boost.com/june2004.html

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[Ichiban]

It makes perfect sense that EGT's, and correspondingly cylinder temperatures, would follow a curve up to stoich, top out there and then fall as the mixture is leaned. Obviously, less fuel=less energy=lower temps.
So lean mixtures are responsible for parts damage because they remain burning in the cylinder and cause preignition on the next cycle, effectively raising temperature and pressure to beyond the autoignition point of the a/f mixture? That's a pretty complicated way to end up with detonation.

In regards to exposure time to the flame front, which I assume you mean, isn't that dictated by how fast the engine is running?

I also want to know why inducing a vacuum leak will raise engine idle Rpm.

[cygnus x-1]

So far, not one of you guys has even come close to refuting what I've said about leaner isn't hotter than stoich, (except for the chainsaw thing, which I think is flawed and will get into later) but causes engine damage by burning more slowly, still at a high temp. Yet you guys keep arguing with that. Evidence of thermal damage doesn't necessarily correspond to a higher temperature. It's temp vs Time of exposure.

I'm not refuting it because I believe that very lean mixtures do not burn as hot as a stoich mixture.

As far as specifically how running lean causes engine damage, I haven't gotten into that yet, now I will. Primarily I would say it leads to detonation. There are many misconceptions in this post about lean a/f mixtures and detonation, a lean mix does not have a lower auto ignition point, it is Not more susceptible to detonation in itself. It doesn Not burn more quickly. It burns slower, and it burns hot, but not hotter than stoich, to the best of my knowledge.

Lean mixtures burn slower and are harder to ignite. And if you go too lean it becomes so difficult to ignite that it won't and you get misfires. I experienced lean misfires when trying to run 17:1 AFR with my megasquirt.

It is belief that this increased exposure time at high temp is what heats up everything in the cylinder more, esp the spark plug and anything susceptible, like sharp edges in the chamber, and gets that glowing exhaust valve even hotter, since it's hotter passing through it, since it's either still burning or has just stopped, unlike a stoich misture, which has stopped by the time that valve opens. Then I would say this increased heat left behind in the chamber causes the fuel to ignite before the plug fires. This causes the entire a/f charge to autoignite at once (detonation) just like if you advanced your ign timing. What I see happening is that the a/f is trying to axpand while the piston is still rising, and the reduction in chamber volume vs the burning mixture compresses the mixture to where it reaches it's autoignition point, like a diesel, and it explodes at once. If the ign point is later, you don't get the 2 forces fighting each other as much, and the a/f is able to burn like normal.

This is almost exactly what I've been thinking but not finding any references to support. Here is some more interesting reading:

http://ballisticmotorsports.org/foru...num=1186877915
http://en.wikipedia.org/wiki/Detonation
http://en.wikipedia.org/wiki/Deflagration


I think I'm going to buy John Heywood's book on combustion engine fundamentals. It doesn't go into more advanced stuff like lean burn engines but it does have all the fundamental theory.

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[cygnus x-1]

In regards to exposure time to the flame front, which I assume you mean, isn't that dictated by how fast the engine is running?

I think yes. This would explain why heavy throttle at low RPMs is hard on engines. With light throttle and low RPM there isn't much air/fuel charge in the cylinder so temperatures won't be so high. But if you go full throttle at low RPM you end up with a full cylinder of air/fuel charge and lots of exposure time. Prime conditions for detonation and engine damage.

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[cygnus x-1]

Here we go!!

http://books.google.com/books?id=Yn9...ode70h2cv0QHLc

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